1. Field of the Invention
The present invention relates to an ion exchange membrane electrolytic cell. More particularly, it relates to an ion exchange membrane electrolytic cell suitable for an electrolysis of water or an aqueous solution of an acid, a base, an alkali metal sulfate, an alkali metal carbonate, or an alkali metal halide.
2. Description of the Prior Art
As a process for producing an alkali metal hydroxide by an electrolysis of an aqueous solution of an alkali metal chloride, a diaphragm method has been mainly employed instead of a mercury method in view of a prevention of a public pollution.
It has been proposed to use an ion exchange membrane in place of asbestos as a diaphragm to produce an alkali metal hydroxide by electrolyzing an aqueous solution of an alkali metal chloride so as to obtain an alkali metal hydroxide having high purity and high concentration.
On the other hand, it has been proposed to save energy in the world. From the viewpoint, it has been required to minimize a cell voltage in such technology.
It has been proposed to reduce a cell voltage by improvements in the materials, compositions and configurations of an anode and a cathode and compositions of an ion exchange membrane and a kind of ion exchange group.
It has been proposed to attain an electrolysis by a so-called solid polymer electrolyte type electrolysis of an alkali metal chloride wherein a cation exchange membrane of a fluorinated polymer is bonded with gas-liquid permeable catalytic anode on one surface and a gas-liquid permeable catalytic cathode on the other surface of the membrane (British Pat. No. 2,009,795, U.S. Pat. Nos. 4,210,501, 4,214,958 and 4,217,401).
This electrolytic method is remarkably advantageous as an electrolysis at a lower cell voltage because an electric resistance caused by an electrolyte and an electric resistance caused by bubbles of hydrogen gas and chlorine gas generated in the electrolysis, can be remarkably decreased which have been considered to be difficult to reduce in the conventional electrolysis.
The anode and the cathode in this electrolytic cell are bonded on the surface of the ion exchange membrane to be embedded partially. The gas and the electrolyte solution are readily permeated so as to easily remove, from the electrode, the gas formed by the electrolysis at the electrode layer contacting with the membrane. Such porous electrode is usually made of a thin porous layer which is formed by uniformly mixing particles which act as an anode or a cathode with a binder, further graphite or the other electric conductive material. However, it has been found that when an electrolytic cell having the electrode bonded directly to an ion exchange membrane is used, the anode in the electrolytic cell is brought into contact with hydroxyl ion which is reversely diffused from the cathode compartment, and accordingly, both of chlorine resistance and an alkaline resistance for anode material are required and an expensive material must be used. When the electrode layer is bonded to the ion exchange membrane, a gas is formed by the electrode reaction between an electrode and membrane and certain deformation phenomenon of the ion exchange membrane is caused to deteriorate the characteristics of the membrane. It is difficult to work for a long time in stable. In such electrolytic cell, the current collector for electric supply to the electrode layer bonded to the ion exchange membrane should closely contact with the electrode layer. When a firm contact is not obtained, the cell voltage may be increased. The cell structure for securely contacting the current collector with the electrode layer is disadvantageously complicated.
The inventors have studied to operate an electrolysis of an aqueous solution at a minimized load voltage and have found that the purpose has been satisfactorily attained by using a cation exchange membrane having a gas and liquid permeable porous non-electrode layer on at least one of surfaces of the cation exchange membrane facing to an anode or a cathode which is proposed in European Patent Publication No. 0029751 or U.S. Ser. No. 205,567.
The effect for reducing a cell voltage by the use of the cation exchange membrane having such porous layer on the surface is depending upon a kind of the material, a porosity and a thickness of the porous layer. Thus, it is surprising phenomenon that the effect for reducing a cell voltage is attained even by the use of the porous layer made of a non-conductive material. The effect for reducing a cell voltage is also attained even though electrodes are placed with a gap from the membrane without contacting the electrode to the membrane, although the extent of the effect is not remarkable.
The electrolytic cell of the invention in which such a porous non-electrode layer is used, is advantageous over a conventional electrolytic cell in which a porous electrode layer is used, in that not only a low cell voltage is thereby obtainable, but also the electrode material can be selected from a wide range of materials since the electrode is not directly in contact with the membrane, and it is thereby possible to avoid troubles due to the generation of gases at the interface between the membrane and the porous layer.
In the electrolytic cell in which such a porous non-electrode layer is used, the uniformity of the porous non-electrode layer and the secure bonding of the layer to the ion exchange membrane are important factors influential to the efficiency of the electrolytic cell. Namely, if the thickness of the porous layer is not uniform or the bonding of the porous layer to the membrane is inadequate, the porous layer tends to be peeled off partly, thus leading to an increase of the cell voltage, or gases or an excess amount of the electrolytic solution tends to be retained at the bonding interface, thus leading to an increase of the cell voltage, whereby the intended advantages tend to be reduced or hardly obtainable.